Séminaires du mardi 03 avril

Séminaire du LPTMS: Raffaela Cabriolu


Creep response of a soft glass

Raffaela Cabriolu (Department of Chemistry, Norwegian University of Science and Technology, Trondheim, Norway)

In this work we discuss finite size effects in the fluidization process of dense amorphous mate- rials subjected to an external load. By means of molecular dynamics simulations we study the mechanical response of a densly packed 3D particle system to a sudden applied shear stress. In order to disentangle possible boundary effects from finite size effects, we use an unusual setup by implementing a geometry-constraint protocol with periodic boundary conditions in all directions. We show that this protocol is well controlled and that the long time fluidization process is to a great extend independent of the details of the protocol parameters. This procedure allows for a robust study of finite size effects regarding the creep exponents and the fluidization process. The slow dynamics show a power-law creep with exponents that do not depend on the system size whereas the fluidisation time shows strong finite size effects, that we can rationalize within a finite size scaling relation.  

Séminaire du LPTMS: Mehdi Bouzid


Athermal analogue of sheared colloidal suspensions

Mehdi Bouzid (LPTMS, Université Paris-Sud)

Sand-piles, window glass, tomato ketchup, are three materials that would not necessarily strike the larger public for their similarities. However, they take part of one of the most lingering enigma of condensed matter physics as they are examples of fluids undergoing dynamical arrest and becoming solid in a way essentially different from a thermodynamic phase transition. Such complex fluids, developing a yield-stress and becoming very hard solids (metallic or oxide glasses) or soft glassy materials (colloidal pastes, granular packing, polymer melts ...), are of central importance in statistical physics, material science or chemical engineering. In this talk I will highlight an analogy between the rheology of Brownian and  non-Brownian  suspensions, we show that these systems can be described by a Herschel-Bulkley law as soon as the shear rate and shear stress  are respectively normalized by an energy scale and a microscopic time of reorganization, which are both functions of the normal confinement stress. The pressure-controlled approach, originally developed for granular flows, reveals a striking physical analogy between the colloidal glass transition and the granular jamming transition.